Wnt genes encode a large family of secreted signaling molecules that play essential roles in animal development and in human cancer and diseases. One of the key Wnt signaling pathways is the canonical b-catenin pathway. Deregulation of Wnt/b-catenin signaling is linked to human colorectal cancers, osteoporosis, and other types of diseases. Wnt/b-catenin signaling is initiated by Wnt activation of two distinct families of cell surface receptors. One is a member of the Frizzled (Fz) family of serpentine receptors, and the other is a single transmembrane receptor belonging to the LDL receptor related protein family, LRP5 or LRP6. How Wnt leads to the activation of these two classes of receptors is a critical but poorly understood issue. We showed that Fz and LRP5/6 can form a complex in the presence of Wnt, and suggested that the Fz-LRP5/6 co-receptor complex triggers intracellular signaling. We also identified Dickkopf-1 (Dkk-1), a known antagonist for Wnt signaling, as a high affinity ligand for LRP5/6, and demonstrated that Dkk-1 disrupts Wnt-induced Fz-LRP5/6 complex formation. Recent human genetic studies revealed important roles of Wnt, Dkk-1 and LRP5 interactions in osteoporosis and high bone density diseases. In this grant application, we propose 3 specific aims to study the molecular basis for Wnt, Dkk, Fz and LRP interactions in Wnt signal transduction: (1) to further investigate Wnt-LRP6-Fz interaction, including Wnt-LRP ligand-receptor binding affinity, identification of Wnt subdomains that bind either LRP or Fz, and potential specificities of Wnt-LRP-Fz interactions. (2) to investigate the interplay between Wnt-LRP and Dkk- LRP interactions and their involvement in human diseases, including mapping LRP domains that bind Wnt and/or Dkk-1, similarities and distinctions between LRP5 and LRP6, the molecular basis of LRP5 mutations in human diseases, and potential functions of a novel Dkk-1 binding protein in Dkk-LRP interaction. (3) to investigate the signaling mechanism of LRP intracellular domain, including the identification of key signaling motifs, and phosphorylation-dependent regulation of LRP function and interaction with intracellular signaling components. We believe that this comprehensive study will provide significant insights into Wnt-LRP interaction in development and diseases. [unreadable] [unreadable]